Probe Of New Physics Research Articles

Azimuthal Angular Correlation as a Boosted Top Jet Substructure.

We propose a novel jet substructure observable of boosted tops that is related to the linear polarization of the W boson in boosted top quark decay, which results in a cos2ϕ angular correlation between the t→bW and W→ff^{'}[over ¯] decay planes. We discuss in detail the origin of such linear polarization by applying Wigner's little group transformation. We show that the unique cos2ϕ angular correlation only exists in the boosted regime but not in the top quark rest frame. We construct an experimental observable for such correlation based on the transverse energy deposition asymmetry in the top jet that does not require the reconstruction of W decay products. The degree of this asymmetry can be used to measure the longitudinal polarization of the top quark, which is an important probe of new physics that couples to the top sector, and can discriminate a boosted top quark jet from its background events, such as QCD jets. A numerical simulation is also performed and found to agree well with the analytic prediction of the standard model.

Searches for new phenomena in final states involving leptons and jets using the ATLAS detector

Many beyond the Standard Model signatures predict new physics phenomena that lead to final states containing both leptons and jets. The latest search results for massive particles or lepton flavor violation using the full Run 2 dataset recorded by the ATLAS experiment are presented. A particular focus is given to searches for leptoquarks, that offer an attractive potential explanation for the lepton flavor anomalies observed at [Formula: see text]-physics factories; searches for heavy leptons, such as the ones appearing in Type-III seesaw model that can explain the smallness of neutrino masses; and lepton flavor violating [Formula: see text] decays, which is an excellent probe for new physics that predict direct lepton flavor violating processes.

Reading the lattice QCD form factors of the Λ b → Λ c transition using a C-code

The process has been discussed in the literatures as a probe for new physics. This process receives contributions in new physics models from scalar, vector, and tensor hadronic currents. The form factors of these currents can be obtained from the quark model or the lattice QCD. In this work, we present a c-code to read the scalar, vector, and tensor lattice QCD form factors. The code is published on github (https://github.com/darkfiresmith96/Lattice_QCD). A Mathematica code was used in reference Datta et al (2017 J. High Energy Phys. 8 131) to read the form factors. Our c-code is much faster than the Mathematica code where the ratio of the wall clock time for the c-code compared to the Mathematica code is 1:64.2 per data point. A web interface for user inputs is introduced.

Capture of primordial black holes in extrasolar systems

The vast datasets associated with extrasolar systems promise to offer sensitive probes of new physics in the near future. We consider the possibility that such systems may capture primordial black holes (PBHs) or other exotic compact objects, giving rise to unique observational signatures. We estimate the rate of captures by extrasolar systems, accounting for several distinct mechanisms. We find that the capture rate is negligible unless PBHs account for the entirety of dark matter in a narrow mass range just above the threshold of existing constraints from evaporation. In this scenario, luminous evaporating PBHs may be detectable by exoplanet searches.

Measurements of μ → 3e decay with polarised muons as a probe of new physics

Working within the Standard Model effective field theory approach, we examine the possibility to test charged lepton flavour violating (cLFV) new physics (NP) by angular measurements of the outgoing electron and positrons in the polarised μ+→e+e+e− decay. This decay is planned to be studied with a sensitivity to the branching ratio of Bμ→3e∼10−15 by the Mu3e experiment at PSI. To illustrate the potential of these measurements, we consider a set of eight effective operators generating the decay at the tree level, including dimension-five dipole operators and dimension-six scalar and vector four-fermion operators of different chiralities. We show that if the polarised μ+→e+e+e− decay is observed and is induced by a single operator, data from three angular observables – two P-odd asymmetries and one T-odd asymmetry – can allow to discriminate between all considered operators with the exception of scalar and vector operators of opposite chirality. For these two types of operators, the P-odd asymmetry is maximal in magnitude; they can be distinguished, in principle, by measuring the helicities of the outgoing positrons. The observation of a non-zero T-odd asymmetry would indicate the presence in the decay rate of a dipole and vector operator interference term that is T (CP) violating.

Fundamental physics with ESPRESSO: Constraints on Bekenstein and dark energy models from astrophysical and local probes

Dynamical scalar fields in an effective four-dimensional field theory are naturally expected to couple to the rest of the theory's degrees of freedom, unless some new symmetry is postulated to suppress these couplings. In particular, a coupling to the electromagnetic sector will lead to spacetime variations of the fine-structure constant, $\alpha$. Astrophysical tests of the space-time stability of $\alpha$ are therefore a powerful probe of new physics. Here we use ESPRESSO and other contemporary measurements of $\alpha$, together with background cosmology data, local laboratory atomic clock and Weak Equivalence Principle measurements, to place stringent constraints on the simplest examples of the two broad classes of varying $\alpha$ models: Bekenstein models and quintessence-type dark energy models, both of which are parametric extensions of the canonical $\Lambda$CDM model. In both cases, previously reported constraints are improved by more than a factor of ten. This improvement is largely due to the very strong local constraints, but astrophysical measurements can help to break degeneracies between cosmology and fundamental physics parameters.

Gearing up for the next generation of LFV experiments, via on-shell methods

Lepton Flavor Violating (LFV) observables such as μ → eγ, μ → 3e and μN → eN are among the best probes for new physics at the TeV scale. In the near future the bounds on these observables will improve by many orders of magnitude. In this work we use the SM EFT to understand the impact of these measurements. The precision reach is such that the interpretation of the bounds requires an analysis of the dimension-six operator mixing up to the two-loop level. Using on-shell amplitude techniques, which make transparent many selection rules, we classify and calculate the different operator mixing chains. At the leading order, on-shell techniques allow to calculate anomalous dimensions of SM EFT operators from the product of tree-level amplitudes, even for two-loop renormalization group mixings. We illustrate the importance of our EFT approach in models with extra vector-like fermions.

Multi-lepton probes of new physics and lepton-universality in top-quark interactions

We explore the sensitivity to new physics (NP) in the associated production of top-quarks with leptons pp→tt¯ℓ+ℓ−, which leads to the multi-leptons signals pp→nℓ+jets+E̸T, where n=2,3,4. The NP is parameterized via 4-Fermi effective tt¯ℓ+ℓ− contact interactions of various types, which are generated by multi-TeV heavy scalar, vector or tensor exchanges in tt¯→ℓ+ℓ−; we focus on the case of ℓ=e,μ. We match the 4-Fermi ttℓℓ terms to the SMEFT operators and also give examples of specific underlying heavy physics that can generate such terms. Analysis of the SM signals and corresponding backgrounds shows that the di-lepton and tri-lepton channels are much better probes of the effective tt¯ℓ+ℓ− 4-Fermi terms than the four-lepton one at the 13 TeV LHC. Therefore, the best sensitivity is obtained in the di- and tri-lepton channels, for which the dominant background pp→tt¯ and pp→WZ, respectively, can be essentially eliminated after applying the 2ℓ and 3ℓ selections and a sufficiently high invariant mass selection for the opposite sign same flavor (OSSF) lepton-pair. We explore two cases: lepton flavor universal (LFU) NP where the ttee and ttμμ contact interactions are of same size and LFU violating (LFUV) NP, where the scale of the ttμμ terms is assumed to be much lower. We show that in both cases it is possible to obtain new 95% CL bounds on the scale of the ttℓℓ contact interactions at the level Λ≳2−3 TeV, which are considerably tighter than the current bounds on these 4-Fermi terms.

Muon electric dipole moment as a probe of flavor-diagonal CP violation

Electric dipole moments (EDMs) of elementary particles are powerful probes of new physics with flavor-diagonal CP violation. The reported discrepancy in the muon anomalous magnetic moment motivates us to explore to what extent new physics with flavor-diagonal CP violation to address the discrepancy is probed by searches for the muon EDM. As a benchmark, we focus on a CP-violating two-Higgs-doublet model to explain the muon g−2 anomaly where the muon exclusively couples to one Higgs doublet. Since contributions to flavor violating processes as well as the electron EDM are suppressed, the muon EDM becomes an essential probe of the model. Our result shows that some viable parameter space leads to the muon EDM of around dμ≃6×10−23e cm probed by the PSI experiment and most of the parameter space is covered by the proposed J-PARC experiment.

Methodologies to Measure the CP Structure of the Higgs Yukawa Coupling to Tau Leptons

One of the central goals of the Large Hadron Collider has been the search, and later the study, of the Higgs boson. Its coupling structure under charge conjugation and parity (CP) symmetries has been extensively investigated as a probe for new physics. This paper presents a review of the methods collected in the literature to access the CP structure of the Yukawa coupling of the Higgs boson to tau leptons at proton colliders. A new notation is introduced to classify already existing methods, highlighting their common features and favoring the investigation of new, more performing alternatives.

Semileptonic tau decays beyond the Standard Model

Hadronic τ decays are studied as probe of new physics. We determine the dependence of several inclusive and exclusive τ observables on the Wilson coefficients of the low-energy effective theory describing charged-current interactions between light quarks and leptons. The analysis includes both strange and non-strange decay channels. The main result is the likelihood function for the Wilson coefficients in the tau sector, based on the up-to-date experimental measurements and state-of-the-art theoretical techniques. The likelihood can be readily combined with inputs from other low-energy precision observables. We discuss a combination with nuclear beta, baryon, pion, and kaon decay data. In particular, we provide a comprehensive and model-independent description of the new physics hints in the combined dataset, which are known under the name of the Cabibbo anomaly.

Neutrino up-scattering via the dipole portal at forward LHC detectors

The significant neutrino flux at high rapidity at the LHC motivates dedicated forward detectors to study the properties of neutrinos at TeV energies. We investigate magnetic dipole interactions between the active neutrinos and new sterile states at emulsion and liquid argon experiments that could be located in a future Forward Physics Facility (FPF) downstream of the ATLAS interaction point. The up-scattering of neutrinos off electrons produces an electron recoil signature that can probe new regions of parameter space at the High Luminosity LHC (HL-LHC), particularly for liquid argon detectors due to low momentum thresholds. We also consider the decay of the sterile neutrino through the dipole operator, which leads to a photon that could be displaced from the production vertex. FPF detectors can test sterile neutrino states as heavy as 1 GeV produced through the dipole portal, highlighting the use of high energy LHC neutrinos as probes of new physics.

Higgs boson decay into a lepton pair and a photon: A roadmap to the discovery of H→Zγ and probes of new physics

The decay $H\ensuremath{\rightarrow}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}\ensuremath{\gamma}$, $\ensuremath{\ell}=e$, $\ensuremath{\mu}$ receives contributions from $H\ensuremath{\rightarrow}Z[\ensuremath{\rightarrow}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}]\ensuremath{\gamma}$ and a nonresonant contribution, both of which are loop induced. We describe how one can separate these subprocesses in a gauge-independent way, define the decay rate $\mathrm{\ensuremath{\Gamma}}(H\ensuremath{\rightarrow}Z\ensuremath{\gamma})$, and extract the latter from differential $H\ensuremath{\rightarrow}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}\ensuremath{\gamma}$ branching ratios. For $\ensuremath{\ell}=\ensuremath{\mu}$, also the tree decay rate, which is driven by the muon Yukawa coupling, is important. We propose kinematic cuts optimized to separate the three contributions, paving the way to the milestone (i) discovery of $H\ensuremath{\rightarrow}Z\ensuremath{\gamma}$, (ii) discovery of $H\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\gamma}|}_{\text{tree}}$, and (iii) quantification of new physics in both the effective $H\text{\ensuremath{-}}Z\text{\ensuremath{-}}\ensuremath{\gamma}$ and nonresonant $H\text{\ensuremath{-}}{\ensuremath{\ell}}^{+}\text{\ensuremath{-}}{\ensuremath{\ell}}^{\ensuremath{-}}\text{\ensuremath{-}}\ensuremath{\gamma}$ couplings.

Status of the Fermilab Muon g – 2 Experiment

The muon anomalous magnetic moment, aμ = (g – 2)/2, is a low-energy observable which can be both measured and computed with very high precision, making it an excellent test of the Standard Model (SM) and a sensitive probe of new physics. Recent efforts improved the precision of both the theoretical prediction and the experimental measurement. On the theory side, the Muon g – 2 Theory Initiative, an international team of more than 130 physicists, reached in 2020 a consensus on the SM prediction for aμ. On the experimental side, the E989 Muon g – 2 Collaboration at Fermilab (FNAL) published in April 2021 a new measurement of aμ from the Run-1 dataset (2018) with 0.46 ppm precision, corroborating the previous Brookhaven National Laboratory (BNL) measurement and increasing the discrepancy with the SM value to 4.2 standard deviations. In this paper, I will discuss the experimental setup and report on the current status of the experiment.

FCC-ee: physics motivations

Learning from the first twelve years of LHC running, this essay offers a brief journey through the FCC-ee physics programme from refined precision measurements to probes of new physics, highlighting some of the commentaries between the different runs of FCC-ee at various energies as well as the synergies between the two FCC-ee and FCC-hh collider stages.

Two-loop electroweak corrections to the Top-Quark Contribution to \u03f5K

The parameter ϵK measures CP violation in the neutral kaon system. It is a sensitive probe of new physics and plays a prominent role in the global fit of the Cabibbo-Kobabyashi-Maskawa matrix. The perturbative theory uncertainty is currently dominated by the top-quark contribution. Here, we present the calculation of the full two-loop electroweak corrections to the top-quark contribution to ϵK, including the resummation of QED-QCD logarithms. We discuss different renormalization prescriptions for the electroweak input parameters. In the traditional normalization of the weak Hamiltonian with two powers of the Fermi constant GF, the top-quark contribution is shifted by −1%.

Z lepton flavour violation as a probe for new physics at future e^+e^- colliders

In this work we assess the potential of discovering new physics by searching for lepton-flavour-violating (LFV) decays of the Z boson, Zrightarrow ell _i ell _j, at the proposed circular e^+e^- colliders CEPC and FCC-ee. Both projects plan to run at the Z-pole as a “Tera Z factory”, i.e., collecting {mathcal {O}}left( 10^{12} right) Z decays. In order to discuss the discovery potential in a model-independent way, we revisit the LFV Z decays in the context of the Standard Model effective field theory and study the indirect constraints from LFV mu and tau decays on the operators that can induce Zrightarrow ell _i ell _j. We find that, while the Zrightarrow mu e rates are beyond the expected sensitivities, a Tera Z factory is promising for Zrightarrow tau ell decays, probing New Physics at the same level of future low-energy LFV observables.

A tale of invisibility: constraints on new physics in b \u2192 s\u03bd\u03bd

The Belle II experiment will measure the rare decays B → Kνν and B → K∗νν with increased sensitivity which can hence be expected to serve as a very efficient probe of new physics. We calculate the relevant branching ratios in low-energy effective field theory (LEFT) including an arbitrary number of massive sterile neutrinos and discuss the expected sensitivity to the different operators. We also take into account the longitudinal polarisation fraction FL and the inclusive decay rate B → Xsνν. In our investigation we consider new physics dominantly contributing to one and two operators both for massless and massive (sterile) neutrinos. Our results show a powerful interplay of the exclusive decay rates B → Kνν and B → K∗νν, and a surprisingly large sensitivity of the inclusive decay mode to vector operators even under conservative assumptions about its uncertainty. Furthermore, the sensitivity of FL is competitive with the branching ratio of B → K∗νν in the search for new physics contributing to scalar operators and thus also complementary to B → Kνν and B → Xsνν.

New physics through Drell-Yan standard model EFT measurements at NLO

Drell Yan production is a sensitive probe of new physics and as such has been calculated to high order in both the electroweak and QCD sectors of the standard model, allowing for precision comparisons between theory and data. Here we extend these calculations to the standard model effective field theory and present the next-to-leading order QCD and electroweak contributions to the neutral Drell-Yan process.

Multi-Messenger Cosmological Probes for New Physics in the Light of Ya.B. Zeldovich’s Legacy

Multi-Messenger Cosmological Probes for New Physics in the Light of Ya.B. Zeldovich’s Legacy